This invention relates to an electronic governor of a fuel supplying device for an engine.
As disclosed in Japanese Laid-Open Patent Publication No. 190639/85, a fuel supplying device for supplying fuel to a diesel engine comprises a fuel injection pump and an electronic governor. The fuel injection pump has an adjustment member as indicated by the reference numeral 7 in FIG. 1 of the above Publication (this numeral 7 as well as other numerals in FIG. 1 of the above Publication will be used in the following description). An injection amount of fuel from the fuel injection pump is determined by a position of the adjustment member. The electronic governor is operated to control the position of the adjustment member 7. The electronic governor comprises a position detection circuit 11 for detecting the position of the adjustment member 7, a rotational speed detection circuit 4 for detecting a rotational speed of the engine, and an accelerator operation amount detection circuit 3 for detecting an accelerator operation amount.
The electronic governor of the above Publication further comprises a target position arithmetic circuit 5, and position control circuits 8 and 9. The position control circuit is operated to control the position of the adjustment member 7 such that the position of the adjustment member 7 detected by the position detection circuit 11 is coincident with a target position calculated by the target position arithmetic circuit 6.
The target position arithmetic circuit 6 comprises a primary arithmetic circuit 12. The primary arithmetic circuit is operated to calculate a primary target position of the adjustment member such that the supply amount of fuel is reduced as the rotational speed of the engine is increased in accordance with predetermined relations between the rotational speed of the engine and the position of the adjustment member. As shown in FIG. 2 of the above Publication, the predetermined relations between the rotational speed of the engine and the position of the adjustment member are represented by different lines which are drawn for different accelerator operation amounts respectively and the target position of the adjustment member is shifted in a fuel supply amount increasing direction as the accelerator operation amount is increased. When the accelerator operation amount is not zero (in other words, when an idling operation is not undergoing), the primary target position is served, in principle, as a final target position.
When the idling control is undergoing, the electronic governor of the above Publication controls the adjustment member such that the rotational speed of the engine becomes constant. As a construction for achieving the foregoing, the target position arithmetic circuit 6 comprises a PI arithmetic circuit and an adder circuit 17 which co-act with the primary arithmetic circuit 12. This PI arithmetic circuit 16 performs a PI calculation in accordance with a deviation between a constant target idling rotational speed and the rotational speed detected by the rotational speed detection circuit. By adding the result of the PI calculation to the primary target position, the adder circuit 17 corrects the primary target position of the adjustment member in the fuel supply amount increasing direction.
The PI arithmetic circuit 16 normally outputs a zero or plus calculation result. Therefore, when the accelerator pedal is released to bring the accelerator operation amount to zero at the time the accelerator operation amount is large, the function of the PI arithmetic circuit 16 is substantially stopped. More specifically, at the time when the accelerator pedal is released, a rotational speed to be detected is still on a higher level than the target idling rotational speed. Therefore, provided that the PI arithmetic circuit can output a minus calculation result, the calculation result at the time when the accelerator pedal is released is increased in a minus direction. If the primary target position is corrected using this calculation result, the rack position is extremely shifted in the fuel supply amount reducing direction, with the result that an undershoot of the fuel injection amount occurs.
The primary target position (hereinafter referred to as the "base position") obtainable at the target idling rotational speed with reference to the line indicative of the relations between the engine rotational speed and the adjustment member position is somewhat shifted in the fuel supply amount reducing direction from the position of the adjustment member required for maintaining the target idling rotational speed. As a result, when the idling control is undergoing, the PI arithmetic circuit 16 is normally effectively functioned. Since the result of the PI calculation is normally added to the base position, the engine rotational speed can be maintained to the target idling rotational speed.
However, when the fuel supplying device of the above Publication is used for a long period of time, it gives rise to such a problem that at the time when the idling control is undergoing, the engine rotational speed becomes unstable. This problem is guessed to be occurred because of the following reasons. That is, the engine is provided with injection nozzles for injecting high pressure fuel discharged from the fuel injection pump into engine cylinders. Each injection nozzle comprises a needle valve for opening and closing an injection port, and a nozzle spring for biasing the needle valve in a closing direction. When fuel pressure from the fuel injection pump becomes higher than a valve opening pressure defined by the nozzle spring, the needle valve opens its injection port to inject the fuel into the engine cylinder. When the injection nozzle is used for a long period of time, the nozzle spring is deteriorated with the passage of time and as a result, the valve opening pressure is lowered. When the valve opening pressure of the nozzle is lowered, the fuel injection amount is increased because the injection portion is open for a long period of time. As a result, when the adjustment member is at the base position, a larger amount of fuel than the fuel injection amount required for maintaining the target idling rotational speed is injected from the fuel injection pump. As a consequence, a satisfactory function of the PI arithmetic circuit 16 is unobtainable, and the target position of the adjustment member is determined chiefly with reference to the line indicative of the relation between the engine rotational speed and the adjustment member position. As a consequence, the engine rotational speed is greatly fluctuated.